1. Field of the Invention
The present invention relates to a PON (Passive Optical Network) system and more particularly to a method for allocating bandwidth used for data transmission to an ONU (Optical Network Unit) in a GE-PON (Gigabit Ethernet Passive Optical Network) system.
2. Description of the Related Art
Because of the growing wide bandwidth multimedia demands and Internet usage, FTTH (Fiber To The Home) techniques for installing optical lines to individual homes and PON (Passive Optical Network) systems have been developed. Typical PON systems have a Point-To-Multipoint access structure that allows a plurality of ONUs (Optical Network Units) to share with an OLT (Optical Line Termination) via one optical fiber. Such PON systems are classified as either an ATM-PON (hereinafter referred to as an APON) system or an Ethernet PON (hereinafter referred to as an EPON) system according to the data transfer method for communicating with subscribers.
The APON system has limitations in the bandwidth range and has a maximum bandwidth of 622 Mbps. The APON also needs to perform IP (Internet Protocol) packet segmentation. In contrast, the EPON system has a high bandwidth of about 1 Gbps. In addition, the PON system has a low production cost so that the EPON system is regarded as being generally superior to the APON system.
FIG. 1 is a block diagram of a conventional EPON system. The EPON system includes one OLT 10, an ODN (Optical Distribution Network) 20, and a plurality of ONUs 30-1, 30-2, . . . , 30-N. The OLT 10 is connected to the ONUs 30-1 to 30-N via the ODN 20. The OLT 10 is located on a tree structure route, and plays an important role in providing each subscriber in an access network with information. The OLT 10 has a tree topology, and is connected to the ODN 20. The ODN 20 distributes downstream data frames received from the OLT 10 to the ONUs 30-1 to 30-N, or multiplexes upstream data frames received from the ONUs 30-1 to 30-N according to a TDM (Time Division Multiplexing) scheme to transmit the multiplexed data frames to the OLT 10. The ONUs 30-1 to 30-N receive downstream data frames, transmit them to end users 40-1, 40-2, . . . , 40-N, and transmit output data of the end users 40-1 to 40-N to the OLT 10 via the ODN 20 as an upstream data frame functioning as a variable length Ethernet frame. The end users 40-1 to 40-N represent a variety of access network termination units available to a PON having an NT (Network Terminal).
A GPS (Generalized Processor Sharing) system is used to share resources between one OLT 10 and ONUs 30-1 to 30-N connected to the OLT 10 in a point-to-multipoint EPON structure. In more detail, the point-to-multipoint EPON system adapts the GPS system to support a fair-queuing algorithm. By applying such a GPS-based algorithm to the EPON system, a scheduler positioned at the OLT schedules a variety of queues positioned at the ONU. However, it is impossible for the scheduler to successfully perform such a fair-queuing algorithm when there is an excessive bandwidth demand.
FIG. 2 depicts a single scheduling process of a GPS-based bandwidth allocation algorithm in the EPON system.
Referring to FIGS. 1 and 2, the ONUs 30-1 to 30-N register in the OLT 10 indicate their existence and their locations. They are then allocated IDs. If the OLT 10 provides the ONUs 30-1 to 30-N with a data transfer opportunity by means of an upstream data transfer opportunity grant frame, the ONUs 30-1 to 30-N recognize the amount of data contained in their queues 32, insert the recognized queue values into a bandwidth allocation request frame, and transmit the resultant frame to the OLT 10. The upstream data transfer opportunity grant frame is a downstream packet for providing one of the ONUs 30-1 to 30-N with an upstream data transfer opportunity. The bandwidth allocation request frame is an upstream packet for enabling one of the ONUs 30-1 to 30-N to send a bandwidth allocation request to the OLT 10 upon receiving a permission message from the OLT 10.
Upon receipt of the bandwidth allocation requests from the ONUs 30-1 to 30-N, the scheduler 12 of the OLT 10 allocates appropriate data transfer bandwidths to the ONUs 30-1 to 30-N. The OLT 10 inserts a bandwidth allocation result received from the scheduler 12 into an upstream data transfer opportunity grant frame of the next timeslot, and transmits the result frame to the ONUs 30-1 to 30-N. Allocation information is composed of first time information indicating a data transfer start time and second time information indicating a data transfer duration time. The ONUs 30-1 to 30-N receiving the first and second time information transmits data to the OLT 20 during their unique allocation times.
The EPON system provides a specific working process with priority information to easily transmit video or audio data. The classification of priority information of Ethernet frames is determined according to 3-bit VLAN (Virtual Local Area Network) tag information of the Ethernet frames and user-defined port priority information. In this way, individual ONUs contain one or more queues allocated to individual subscribers. Multiple queues belonging to individual subscribers are adapted to provide the subscriber with other class or other traffic services such as audio, video, and data services.
The above GPS-based bandwidth allocation algorithms fairly allocate/transmit appropriate bandwidths to individual queues of the ONUs. They have limitations, however, in extensibility and efficiency of an EPON system due to the following three disadvantages (a) to (c).
a) Transmission Delay in System Control Process:
If it is assumed that the algorithms used with the GPS system create a very small transmission delay, then a variation in the queue status is immediately transferred to the scheduler, and the scheduler can immediately recognize a changed packet size caused by the queue status variation. This assumption is true when system or queue of a single chip is close to a scheduler. However, because the EPON system is a distributed system, the transfer delay time between the scheduler and the queues is longer than a packet transmission time. In addition, the ONU performs data transmission only during its own allocation timeslot time so that data of the ONU does not collide with data of other ONUs. Accordingly, a REPORT message that indicates queue status variation information must be transmitted over a previously-allocated timeslot. This causes the control message delay to be further increased along with the transmission delay.
b) Limited Control Bandwidth:
GATE messages for every queue and REPORT messages for every queue are needed to perform scheduling on a variety of queues. The GATE messages for every queue must be transferred to individual queues, and the REPORT messages for every queue must transmit bandwidths requested by individual queues. For example, if there are 32 ONUs that provide 128 subscribers with desired services and each subscriber has three queues, then 12288 queues are created. If one queue of the three queues provides video data service having a maximum delay of 1.5 ms, 4096 GATE messages must be transmitted within a prescribed range of 1.5 ms. However, a predetermined time of 2.75 ms is consumed to transmit such GATE messages, such that queues for 4096 video services cannot be supported due to the limited control bandwidth.
c) Switch-Over Overhead:
A packet-based GPS system schedules packets on the basis of its virtual termination time. Virtual time information is affected by packet reception time and relative weights for every access point. It is noted, however, that this operation can be performed when packets are successively received from other queues positioned at other ONUs. The EPON system requires a guard time between packets transferred from other queues positioned at other ONUs. Therefore, in the case of considering a mean Ethernet packet size of 500 bytes, link capacity of 1 Gbps, and a guard time of 1 μs, an overhead of 20% is created. If a minimum Ethernet packet size of 64 bytes is transmitted, an overhead of 66% is created.
In conclusion, the aforementioned conventional packet-based GPS algorithm has significant shortcomings for use with EPON systems.